Digital Audio for Hifi
Many hifi listeners have opted to use an external DAC, or digital to analog converter, instead of the one built into their CD player. The reason for this could be that they only have a CD transport hence the only output is digital or they wanted to improve the performance of the DAC stage in the player by using a newer or better external one. Of course not all external DAC’s are as good as internal ones, it depends on the design and componentry used. But the main purpose of this article is to look at different ways of connecting the CD transport to the DAC itself.
Types of Digital Outputs
TOSLINK is an optical interface that uses mainly plastic optical cable. High-end versions use glass fibre and of course the more you spend, the better quality the construction and optical characteristics of the cable. It was developed by Japanese company Toshiba but can now be found on a huge range of hifi products. TOSLINK usually has a small square plug or folding cover installed to keep the lens free from dust. You can see it is operating by the red glow.
Coaxial is generally in the form of a standard RCA (or occasionally the better quality BNC) connector and uses coaxial wire as it’s interconnect. An RCA connector is the most common coaxial output connector. Again quality differences occur, as you go up a range, better quality RCA sockets / plugs are used and the cable has better quality materials used during construction.
How it Works
General consensus is that coaxial is preferable over TOSLINK where possible for a few different reasons. Many people claim they cannot hear the difference, this could be due to a poor quality coaxial output using low speed parts and not the correct impedance (75 Ohm) matching also the system must be good enough to resolve the differences. So why does coaxial tend to be better, all else being equal?
TOSLINK utilises an optical connection which you would initially think would be better, but being often based on plastic optical cable it is prone to jitter (the timing of the digital to analog conversion becomes skewed). The electrical coaxial digital interface has lower levels of jitter. Also there is less conversion of the signal with coaxial, with optical it is converted from electrical to light then back again to electrical, with coaxial there is no such optical conversion as it remain electrical the whole way in most cases. (Some high-end coaxial interfaces use small special transformers to obtain the correct impedance.)
So what differences can you expect to hear between different digital signals? Some say that digital is digital, no matter how it is carried. That may be true in regards to the actual data, the ones and zeros may be the same (assuming no losses from very poor quality, faulty or very long cables) but the timing differences between those ones and zeros are very important as well. It is these timing differences that are referred to as jitter. They can cause problems with the actual processing of the digital signal including the all-important digital to analog conversion.
The sound will typically have less staging, less clarity and definition and bass can be muddier. One trouble is measuring jitter, the equipment to measure it costs many thousands and is not easily available so it is difficult to know how much jitter you are experiencing. The main thing is to try and use the best digital cable you can and even experiment between optical and coaxial to see which you prefer. After all, every system is different and ever person hears things differently!